The present invention relates to a method of heating a non-ferrous and/or ferrous metal-containing stock in a furnace with a heating chamber, a charging door, an exhaust stream port and an exhaust stream duct wherein fuel and an oxygen-containing gas are introduced into the furnace so that a flame is formed, and to an apparatus for performing said method. By heating it is meant to include melting, heating, recycling, smelting and otherwise processing metals by application of heat.
Heating of non-ferrous and ferrous metal containing stocks, in particular aluminium containing stocks, in furnaces is well-known in the art. A problem which occurs in these processes is that the composition and quality of the stocks used for heating is usually varying. For example, organic components such as e.g. oils, lacquer, paper, plastics, rubber, paints, coatings etc. may be present in the material to be heated. These organic materials are pyrolized when the volatilisation temperature is attained and, when oxygen is deficient, brought out to the exhaust duct of the furnace as CO or uncombusted hydrocarbons. The gas cleaning systems usually employed are not able to completely eliminate these unwanted noxious substances from the exhaust stream which are, hence, emitted to the environment if no further measures are taken.
In the art, several attempts have been made to improve the combustion efficiency in the furnace so as to lower the emission of the noxious substances to the environment. For example, in U.S. Pat. Nos. 7,462,218, 7,648,558 and 7,655,067 processes are disclosed in which the variation of CO and/or H2 concentration in the exhaust gases and the temperature thereof are measured, and the fuel flow to the furnace is adjusted accordingly.
EP 553 632 discloses a process in which continuously the temperature of the exhaust gas stream from the furnace is measured and, when the temperature exceeds a pre-determined value, the oxygen content in the furnace is increased.
In EP 1 243 663, a process is disclosed in which the O2 content in the exhaust gases of the furnace is measured and this measurement is then used as a guide variable for the control unit.
WO 2004/108975 discloses a process in which the O2 and CO content in the exhaust gases of the furnace are measured and the additional injection of oxygen is controlled using those measurements.
Finally, in EP 756 014, a process is disclosed in which the concentration of hydrocarbons in the exhaust gases from the furnace is measured and the volume of oxygen and/or the volume of fuel introduced into the furnace is set as a function of the measured concentration of said substances.
The disclosure of the previously identified patents and patent applications is hereby incorporated by reference.
In spite of these prior art processes there is still the need for an improved control of heating processes, in particular of the combustion taking place in a heating furnace, in order to minimize the emission of noxious substances, such as CO and hydrocarbons, to the environment, and to increase the overall efficiency of the furnace.
It is therefore the object of the present invention to provide such an improved process, in particular for heating of heavy organic contaminated stocks.